Method and apparatus for sensing field strength signals to estimate location of a wireless implantable marker

ABSTRACT

Embodiments of the invention are directed to an apparatus for use in a system that senses an excitable wireless target capable of being implanted in a body or tissue. The apparatus includes multiple electromagnetic field sensors arranged approximately in a common plane, and multiple sense signal output paths coupled to the sensors. Each one of the sensors and corresponding output paths is configured to provide an output signal representing at least a portion of an electromagnetic field provided by the marker, where the output signal is proportional to a component of the field at the sensor, where that component is substantially perpendicular to the plane. Various other configurations regarding this apparatus, as well as the overall system and methods of exciting and receiving signals from wireless markers, are also disclosed.

TECHNICAL FIELD

Embodiments of the invention relate to systems for sensing miniaturemarkers, such as systems for sensing resonating miniature markerassemblies in tissue for use in healthcare applications.

BACKGROUND

Systems have been developed to activate and detect remote activatablemarker assemblies positioned, for example, in or on a selected object orbody. The markers generate a signal used to detect the presence of themarker. Many of the activatable markers are hard-wired to a power sourceor other equipment external from the object. Other systems have beendeveloped that utilize resonating leadless or “wireless” markers. Thesewireless markers are typically activated or energized by a remoteexcitation source that wirelessly transmits a strong continuous orpulsed excitation signal. In response to the excitation signal, thewireless markers wirelessly transmit a detectable marker signal thatmust be distinguished from the strong excitation signal and thenanalyzed in an effort to try to accurately determine the location of thetarget. The process of distinguishing a weak marker signal from thestrong excitation signal to consistently and accurately determine thelocation of the marker has proven to be very difficult.

One example is U.S. Pat. No. 5,325,873 to Hirschi et al., which teachesa system that detects the general position of an object within a body oftissue. The detection system includes a three-axis resonant-circuittarget attached to the object and a separate hand-held detection probehaving a pair of parallel and coaxially aligned transmitter/sensingcoils. A current is induced in the transmitter/sensing coils thatdetermines whether a return signal strength of the target is sufficientto be counted as a valid signal. The hand-held detection probe also hasa pair of receiver coils positioned within the transmitter coils andconnected in a series-opposed fashion. The receiver coils allow for thecreation of a null circuit condition when the target is equidistant fromeach of the receiver coils. The detection probe also has a visualdisplay coupled to the receiver coils and configured to indicate thedirection (e.g., left/right/up/down) in which the probe should be movedto center the detection probe over the object for achieving the nullcircuit condition.

Further details regarding prior systems may be found in U.S. patentapplication Ser. No. 10/027,675 entitled “System For Excitation Of ALeadless Miniature Marker” filed Dec. 20, 2001 (Attorney Docket No.34114-8006.US00), U.S. patent application Ser. No. 10/044,056 entitled“System For Excitation Of A Leadless Miniature Marker” filed Jan. 11,2002 (Attorney Docket No. 34114-8006.US01), and U.S. patent applicationSer. No. 10/213,980 entitled “System For Spatially Adjustable ExcitationOf Leadless Miniature Marker” filed Aug. 7, 2002 (Attorney Docket No.34114-8006.US02).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an example of a system for estimatingthe location of wireless implantable markers.

FIG. 2 is a block diagram illustrating components of the system of FIG.1 including a sensing subsystem.

FIG. 3A is an exploded isometric view showing individual components of asensing subsystem in accordance with an embodiment of the invention.

FIG. 3B is a top plan view of an example of a sensing assembly of asensing subsystem.

FIG. 4 is a schematic diagram of a suitable preamplifier for use withthe sensing subsystem of FIG. 3.

Sizes of various depicted elements are not necessarily drawn to scale,and these various elements may be arbitrarily enlarged to improvelegibility. Also, the headings provided herein are for convenience onlyand do not necessarily affect the scope or meaning of the claimedinvention.

DETAILED DESCRIPTION

Embodiments of the invention are directed to an apparatus for use in asystem that senses an excitable wireless marker capable of beingimplanted in a body or tissue. The apparatus includes multipleelectromagnetic field sensors arranged in a locally planar array (e.g.,an array in a common plane), and multiple sense signal output pathscoupled to the sensors. The sensors and the corresponding output pathsare configured to provide an output signal representing at least aportion of an electromagnetic field emitted by the marker; the outputsignal from a specific sensor is proportional to the component of thefield that is substantially perpendicular to the plane of the sensorintegrated over its aperture. Various other configurations regardingthis apparatus, as well as the overall system and methods of excitingand receiving signals from wireless markers, are also disclosed.

The invention will now be described with respect to various embodiments.The following description provides specific details for a thoroughunderstanding of, and enabling description for, these embodiments of theinvention. However, one skilled in the art will understand that theinvention may be practiced without these details. In other instances,well-known structures and functions have not been shown or described indetail to avoid unnecessarily obscuring the description of theembodiments of the invention.

Description of Suitable Systems

FIG. 1 is a perspective view showing an example of a system 100 forenergizing and locating one or more wireless markers inthree-dimensional space. The system includes an excitation source andsensor array 102 supported by a movable arm 104. The arm 104 is securedto a base unit 106 that includes various components, such as a powersupply, computer (such as an industrial personal computer), and inputand output devices, such as a display 108. Many of these components aredescribed in detail below.

The system 100 may be used with guided radiation therapy to accuratelylocate and track a target in a body to which guided radiation therapy isdelivered. Further details on use of the system with such therapy may befound in U.S. patent application Ser. No. 09/877,498, entitled “GuidedRadiation Therapy System,” filed Jun. 8, 2001 (Attorney Docket No.34114-8004US00), which is herein incorporated by reference.

FIG. 2 is a block diagram of certain components of the system 100. Inparticular, the excitation source and sensor array 102 includes anexcitation subsystem 202 and a sensing subsystem 204. The excitationsystem 202 outputs electromagnetic energy to excite at least onewireless marker 206, and the sensing system 204 receives electromagneticenergy from the marker. Further details regarding the excitationsubsystem 202 may be found in U.S. patent applications noted above.Details regarding the sensing subsystem 204 are provided below.

A signal processing subsystem 208 provides signals to the excitationsubsystem 202 to generate the excitation signals. In the embodimentdepicted herein, excitation signals in the range of 300 to 500 kilohertzmay be used. The signal processing subsystem 208 also receives signalsfrom the sensing subsystem 204. The signal processing subsystem 208filters, amplifies and correlates the signals received from the sensingsubsystem 204 for use in a computer 210.

The computer 210 may be any suitable computer, such as an industrialpersonal computer suitable for medical applications or environments. Oneor more input devices 212 are coupled to the computer and receive userinput. Examples of such input devices 212 include keyboards,microphones, mice/track balls, joy sticks, etc. The computer generatesoutput signals provided to output devices 214. Examples of such outputdevices include the display device 108, as well as speakers, printers,and network interfaces or subsystems to connect the computer with othersystems or devices.

Unless described otherwise herein, several aspects of the invention maybe practiced with conventional systems. Thus, the construction andoperation of certain blocks shown in FIG. 2 may be of conventionaldesign, and such blocks need not be described in further detail to makeand use the invention because they will be understood by those skilledin the relevant art.

Description of Suitable Sensing Subsystems

FIG. 3A is an exploded isometric view showing several components of thesensing subsystem 204. The subsystem 204 includes a sensing assembly 301having a plurality of coils 302 formed on or carried by a panel 304. Thecoils 302 can be field sensors or magnetic flux sensors arranged in asensor array 305. The panel 304 may be a substantially non-conductivesheet, such as KAPTON® produced by DuPont. KAPTON® is particularlyuseful when an extremely stable, tough, and thin film is required (suchas to avoid radiation beam contamination), but the panel 304 may be madefrom other materials. For example, FR4 (epoxy-glass substrates), GETEKand other Teflon-based substrates, and other commercially availablematerials can be used for the panel 304. Additionally, although thepanel 304 may be a flat, highly planar structure, in other embodiments,the panel may be curved along at least one axis. In either embodiment,the field sensors (e.g., coils) are arranged in a locally planar arrayin which the plane of one field sensor is at least substantiallycoplanar with the planes of adjacent field sensors. For example, theangle between the plane defined by one coil relative to the planesdefined by adjacent coils can be from approximately 0° to 10°, and moregenerally is less than 5°. In some circumstances, however, one or moreof the coils may be at an angle greater than 10° relative to other coilsin the array.

The sensing subsystem 204 shown in FIG. 3A can further include alow-density foam spacer or core 320 laminated to the panel 304. The foamcore 320 can be a closed-cell Rohacell foam. The foam core 320 ispreferably a stable layer that has a low coefficient of thermalexpansion so that the shape of the sensing subsystem 204 and therelative orientation between the coils 302 remains within a definedrange over an operating temperature range.

The sensing subsystem 204 can further include a first exterior cover 330a on one side of the sensing subsystem and a second exterior cover 330 bon an opposing side. The first and second exterior covers 330 a-b can bethin, thermally stable layers, such as Kevlar or Thermount films. Eachof the first and second exterior covers 330 a-b can include electricshielding 332 to block undesirable external electric fields fromreaching the coils 302. The electric shielding, for example, prevents orminimizes the presence of eddy currents caused by the coils 302 orexternal magnetic fields. The electric shielding can be a plurality ofparallel legs of gold-plated, copper strips to define a comb-shapedshield in a configuration commonly called a Faraday shield. It will beappreciated that the shielding can be formed from other materials thatare suitable for shielding. The electric shielding can be formed on thefirst and second exterior covers using printed circuit boardmanufacturing technology or other techniques.

The panel 304 with the coils 302 is laminated to the foam core 320 usinga pressure sensitive adhesive or another type of adhesive. The first andsecond exterior covers 330 a-b are similarly laminated to the assemblyof the panel 304 and the foam core 320. The laminated assembly forms arigid, lightweight structure that fixedly retains the arrangement of thecoils 302 in a defined configuration over a large operating temperaturerange. As such, the sensing subsystem 204 does not substantially deflectacross its surface during operation. The sensing subsystem 204, forexample, can retain the array of coils 302 in the fixed position with adeflection of no greater than ±0.5 mm, and in some cases no more than±0.3 mm. The stiffness of the sensing subsystem 204 provides veryaccurate and repeatable monitoring of the precise location of leadlessmarkers in real time.

The sensing subsystem 204 can also have a low mass per unit area in theplane of the sensor coils 302. The “mass-density” is defined by the massin a square centimeter column through the thickness of the sensingsubsystem 204 orthogonal to the panel 304. In several embodiments, thesensing subsystem 204 has a low-density in the region of the coils 302to allow at least a portion of the sensing subsystem 204 to dwell in aradiation beam of a linear accelerator used for radiation oncology. Forexample, the portion of the sensing subsystem 204 including the coils302 can have a mass density in the range of approximately 1.0 gram/cm²or less. In general, the portion of the sensing subsystem that is toreside in the beam of a linear accelerator has a mass-density betweenapproximately 0.1 grams/cm² and 0.5 grams/cm², and often with an averagemass-density of approximately 0.3 grams/cm². The sensing subsystem 204can accordingly reside in a radiation beam of a linear acceleratorwithout unduly attenuating or contaminating the beam. In one embodiment,the sensing subsystem 204 is configured to attenuate a radiation beam byapproximately only 0.5% or less, and/or increase the skin dose in apatient by approximately 80%. In other embodiments, the panel assemblycan increase the skin dose by approximately 50%. Several embodiments ofthe sensing subsystem 204 can accordingly dwell in a radiation beam of alinear accelerator without unduly affecting the patient or producinglarge artifacts in x-ray films.

In still another embodiment, the sensing subsystem 204 can furtherinclude a plurality of source coils that are a component of theexcitation subsystem 202. One suitable array combining the sensingsubsystem 204 with source coils is disclosed in U.S. patent applicationSer. No. 10/334,700, entitled PANEL-TYPE SENSOR/SOURCE ARRAY ASSEMBLY,filed on Dec. 30, 2002, which is herein incorporated by reference.

FIG. 3B further illustrates an embodiment of the sensing assembly 301.

In this embodiment, the sensing assembly 301 includes 32 sense coils302; each coil 302 is associated with a separate channel 306 (shownindividually as channels “Ch 0 through Ch 31”). The overall dimension ofthe panel 304 can be approximately 40 cm by 54 cm, but the array 305 hasa first dimension D₁ of approximately 40 cm and a second dimension D₂ ofapproximately 40 cm. The coil array 305 can have other sizes or otherconfigurations (e.g., circular) in alternative embodiments.Additionally, the coil array 305 can have more or fewer coils, such as8-64 coils; the number of coils may moreover be a power of 2.

The coils 302 may be conductive traces or depositions of copper oranother suitably conductive metal formed on the KAPTON® sheet. Each coil302 has a trace with a width of approximately 0.15 mm and a spacingbetween adjacent turns within each coil of approximately 0.13 mm. Thecoils 302 can have approximately 15 to 90 turns, and in specificapplications each coil has approximately 40 turns. Coils with less than15 turns may not be sensitive enough for some applications, and coilswith more than 90 turns may lead to excessive voltage from the sourcesignal during excitation and excessive settling times resulting from thecoil's lower self-resonant frequency. In other applications, however,the coils 302 can have less than 15 turns or more than 90 turns.

As shown in FIG. 3B, the coils 302 are arranged as square spirals,although other configurations may be employed, such as arrays ofcircles, interlocking hexagons, triangles, etc. Such square spiralsutilize a large percentage of the surface area to improve the signal tonoise ratio. Square coils also simplify design layout and modeling ofthe array compared to circular coils; for example, circular coils couldwaste surface area for linking magnetic flux from the wireless markers206. The coils 302 have an inner dimension of approximately 40 mm, andan outer dimension of approximately 62 mm, although other dimensions arepossible depending upon applications. Sensitivity may be improved withan inner dimension as close to an outer dimension as possible givenmanufacturing tolerances. In several embodiments, the coils 32 areidentical to each other or at least configured substantially similarly.

The pitch of the coils 302 in the coil array 305 is a function of, atleast in part, the minimum distance between the marker and the coilarray. In one embodiment, the coils are arranged at a pitch ofapproximately 67 mm. This specific arrangement is particularly suitablewhen the wireless markers 206 are positioned approximately 7-27 cm fromthe sensing subsystem 204. If the wireless markers are closer than 7 cm,then the sensing subsystem may include sense coils arranged at a smallerpitch. In general, a smaller pitch is desirable when wireless markersare to be sensed at a relatively short distance from the array of coils.The pitch of the coils 302, for example, is approximately 50%-200% ofthe minimum distance between the marker and the array.

In general, the size and configuration of the coil array 305 and thecoils 302 in the array 305 depend on the frequency range in which theyare to operate, the distance from the wireless markers 206 to the array,the signal strength of the markers, and several other factors. Thoseskilled in the relevant art will readily recognize that other dimensionsand configurations may be employed depending, at least in part, on adesired frequency range and distance from the markers to the coils.

The coil array 305 is sized to provide a large aperture to measure themagnetic field emitted by the markers. It can be particularlychallenging to accurately measure the signal emitted by an implantablemarker that wirelessly transmits a marker signal in response to awirelessly transmitted energy source because the marker signal is muchsmaller than the source signal and other magnetic fields in a room(e.g., magnetic fields from CRTs, etc.). The size of the coil array 305can be selected to preferentially measure the near field of the markerwhile mitigating interference from far field sources. In one embodiment,the coil array 305 is sized to have a maximum dimension D₁ or D₂ acrossthe surface of the area occupied by the coils that is approximately 100%to 300% of a predetermined maximum sensing distance that the markers areto be spaced from the plane of the coils. Thus, the size of the coilarray 305 is determined by identifying the distance that the marker isto be spaced apart from the array to accurately measure the markersignal, and then arrange the coils so that the maximum dimension of thearray is approximately 100%-300% of that distance. The maximum dimensionof the coil array 305, for example, can be approximately 200% of thesensing distance at which a marker is to be placed from the array 305.In one specific embodiment, the marker 206 has a sensing distance of 20cm and the maximum dimension of the array of coils 302 is between 20 cmand 60 cm, and more specifically 40 cm.

A coil array with a maximum dimension as set forth above is particularlyuseful because it inherently provides a filter that mitigatesinterference from far field sources. As such, one aspect of severalembodiments of the invention is to size the array based upon the signalfrom the marker so that the array preferentially measures near fieldsources (i.e., the field generated by the marker) and filtersinterference from far field sources.

For example, consider a circular array of radius r in the plane z=0 withthe center of the array at the origin and a marker located over thearray at {0, 0, z_(b)}. Assume the array is comprised of sensors thatare (a) responsive to the normal component of an incident field and (b)placed with sufficient density that the sensed signals from a dipolarsource can be modeled as spatially continuous across the array. In thelimit of r→∞, the available energy of a dipolar field (i.e., theintegral over the plane of the array of the squared normal field) is

$\begin{matrix}{E_{\infty} = {\mu_{0}^{2}m^{2}\frac{3( {1 + {\cos^{2}\varphi}} )}{128\pi \; z_{b}^{4}}}} & (1)\end{matrix}$

where μ₀ is the permeability of free space (or the permeability of themedia of interest if a free space model is inappropriate), m is themagnitude of the magnetic moment of the source, and φ is the angle ofthe dipole axis from the z axis. The available energy thus drops at 12dB/octave, i.e., for every doubling of distance, the energy drops by afactor of sixteen. For finite r, let ξ=z_(b)/r, in which case theintegrated energy is

$\begin{matrix}{E_{r} = {E_{\infty}\frac{{3( {1 + {4\xi^{2}} + {6\xi^{4}}} )} + {( {3 + {12\xi^{2}} + {14\xi^{4}} + {32\xi^{6}}} )\cos^{2}\varphi}}{3( {1 + \xi^{2}} )( {1 + {\cos^{2}\varphi}} )}}} & (2)\end{matrix}$

For small ξ the available energy over a finite array is essentially thesame as that over an infinite array, and, as the distance to the sourceincreases, this holds true until ξ≈1. For sources at greater distances,equation (2) asymptotically approaches −6 dB/octave when φ=0, or −12dB/octave when φ=πr/2. Hence an array can be configured topreferentially receive signals from markers which are in its near field(i.e., within approximately 50% of the maximum dimension of the array)compared to sources beyond its near field, which may consist ofinterfering sources of energy. Specifically, energy from a near fieldsource decreases proportionally to the fourth power of distance from thearray, while energy from a far field source decreases asymptotically asthe sixth or eighth power of distance from the array.

Thus, when the wireless marker 206 is positioned approximately 20 cmfrom the sensing subsystem 204, and a radius or maximum dimension of thesensing subsystem is approximately 40 cm, energy from the wirelessmarker drops off as the fourth power of the distance from the sensingsubsystem while environmental noise drops off as the sixth or eighthpower of the distance. The environmental noise is thus filtered by thesensing subassembly 204, by virtue of its geometry, to provide bettersignals to the signal processing subsystem 208.

The size or extent of the array may be limited by several factors. Forexample, the size of the sensing assembly 301 should not be so large asto mechanically interfere with the movable arm 104 (FIG. 1), the baseunit 106 (FIG. 1), or other components, such as a patient couch,rotating gantry of a radiation therapy machine, etc. (not shown in FIG.1). Also, the size of the array may be limited by manufacturingconsiderations, such as a size of available panels 304. Further, makinga dimension or width of the coil array 305 larger than twice thedistance to the wireless marker 206 may yield little performanceimprovement, but increase manufacturing costs and increase sensitivityto interference.

The coils 302 are electromagnetic field sensors that receive magneticflux produced by the wireless marker 206 and in turn produce a currentsignal representing or proportional to an amount or magnitude of acomponent of the magnetic field through an inner portion or area of eachcoil. The field component is also perpendicular to the plane of eachcoil 302. Importantly, each coil represents a separate channel, and thuseach coil outputs signals to one of 32 output ports 306. A preamplifier,described below, may be provided at each output port 306. Placingpreamplifiers (or impedance buffers) close to the coils minimizescapacitive loading on the coils, as described herein. Although notshown, the sensing assembly 301 also includes conductive traces orconductive paths routing signals from each coil 302 to its correspondingoutput port 306 to thereby define a separate channel. The ports in turnare coupled to a connector 308 formed on the panel 304 to which anappropriately configured plug and associated cable may be attached.

The sensing assembly 301 may also include an onboard memory or othercircuitry, such as shown by electrically erasable programmable read-onlymemory (EEPROM) 310. The EEPROM 310 may store manufacturing informationsuch as a serial number, revision number, date of manufacture, and thelike. The EEPROM 310 may also store per-channel calibration data, aswell as a record of run-time. The run-time will give an indication ofthe total radiation dose to which the array has been exposed, which canalert the system when a replacement sensing subsystem is required.

While shown in only one plane, additional coils or electromagnetic fieldsensors may be arranged perpendicular to the panel 304 to help determinea three-dimensional location of the wireless markers 206. Adding coilsor sensors in other dimensions could increase the total energy receivedfrom the wireless markers 206, but the complexity of such an array wouldincrease disproportionately. The inventors have found thatthree-dimensional coordinates of the wireless markers 206 may be foundusing the planar array shown in FIG. 3B.

Description of a Suitable Preamplifier

Implementing the sensing subsystem 204 may involve severalconsiderations. First, the coils 302 may not be presented with an idealopen circuit. Instead, they may well be loaded by parasitic capacitancedue largely to traces or conductive paths connecting the coils to thepreamplifiers, as well as a damping network (described below) and aninput impedance of the preamplifiers (although a low input impedance ispreferred). These combined loads result in current flow when the coils302 link with a changing magnetic flux. Any one sense coil 302, then,links magnetic flux not only from the wireless marker 206, but also fromall the other sense coils as well. These current flows should beaccounted for in downstream signal processing.

A second consideration is the capacitive loading on the coils 302. Ingeneral, it is desirable to minimize the capacitive loading on the coils302. Capacitive loading forms a resonant circuit with the coilsthemselves, which leads to excessive voltage overshoot when theexcitation subsystem 202 is energized. Such a voltage overshoot shouldbe limited or attenuated with a damping or “snubbing” network across thecoils 302. A greater capacitive loading requires a lower impedancedamping network, which can result in substantial power dissipation andheating in the damping network.

Another consideration is to employ preamplifiers that are low noise. Thepreamplification can also be radiation tolerant because one applicationfor the sensing subsystem 204 is with radiation therapy systems that uselinear accelerators (LINAC). As a result, PNP bipolar transistors anddiscrete elements may be preferred. Further, a DC coupled circuit may bepreferred if good settling times cannot be achieved with an AC circuitor output, particularly if analog to digital converters are unable tohandle wide swings in an AC output signal.

FIG. 4, for example, illustrates an embodiment of a snubbing network 402having a differential amplifier 404. The snubbing network 402 includestwo pairs of series coupled resistors and a capacitor bridgingtherebetween. A biasing circuit 406 allows for adjustment of thedifferential amplifier, while a calibration input 408 allows both inputlegs of the differential amplifier to be balanced. The sensor coil 302is coupled to an input of the differential amplifier 404, followed by apair of high voltage protection diodes 410. DC offset may be adjusted bya pair of resistors coupled to bases of the input transistors for thedifferential amplifier 404 (shown as having a zero value). Additionalprotection circuitry is provided, such as ESD protection diodes 412 atthe output, as well as filtering capacitors (shown as having a 10 nFvalue).

CONCLUSION

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,” and thelike are to be construed in an inclusive sense as opposed to anexclusive or exhaustive sense, that is to say, in the sense of“including, but not limited to.” Words using the singular or pluralnumber also include the plural or singular number, respectively.Additionally, the words “herein,” “above,” “below” and words of similarimport, when used in this application, shall refer to this applicationas a whole and not to any particular portions of this application. Whenthe claims use the word “or” in reference to a list of two or moreitems, that word covers all of the following interpretations of theword: any of the items in the list, all of the items in the list, andany combination of the items in the list.

The above detailed descriptions of embodiments of the invention are notintended to be exhaustive or to limit the invention to the precise formdisclosed above. While specific embodiments of, and examples for, theinvention are described above for illustrative purposes, variousequivalent modifications are possible within the scope of the invention,as those skilled in the relevant art will recognize. For example, anarray of hexagonally shaped sense coils may be formed on a planar arraycurved along at least one line to form a concave structure.Alternatively, the arrangement of coils on the panel may form patternsbesides the “cross” pattern shown in FIGS. 3A and 3B. The coils may bearranged on two or more panels or substrates, rather than the singlepanel described herein. The teachings of the invention provided hereincan be applied to other systems, not necessarily the system employingwireless, implantable resonating targets described in detail herein.These and other changes can be made to the invention in light of thedetailed description.

The elements and acts of the various embodiments described above can becombined to provide further embodiments. All of the above U.S. patentsand applications and other references are incorporated herein byreference. Aspects of the invention can be modified, if necessary, toemploy the systems, functions and concepts of the various referencesdescribed above to provide yet further embodiments of the invention.

These and other changes can be made to the invention in light of theabove detailed description. In general, the terms used in the followingclaims should not be construed to limit the invention to the specificembodiments disclosed in the specification, unless the above detaileddescription explicitly defines such terms. Accordingly, the actual scopeof the invention encompasses the disclosed embodiments and allequivalent ways of practicing or implementing the invention under theclaims.

While certain aspects of the invention are presented below in certainclaim forms, the inventors contemplate the various aspects of theinvention in any number of claim forms. For example, while only oneaspect of the invention is recited as embodied in a means plus functionclaim, other aspects may likewise be embodied in a means plus functionclaim. Accordingly, the inventors reserve the right to add additionalclaims after filing the application to pursue such additional claimforms for other aspects of the invention.

1-53. (canceled)
 54. A method of manufacturing a system for sensing andlocalizing a wireless marker configured to wirelessly transmit a markersignal in response to a wirelessly transmitted excitation signal,comprising: selecting a sensing distance at which a sensor array is tobe positioned from the marker in operation based upon the marker signal;and providing a sensing system having a support member and a pluralityof field sensors carried by the support member, the field sensors beingarranged in a locally planar array relative to one another andresponsive to magnetic field components at least substantially normal tothe field sensors, and the field sensors being arranged in an arrayhaving maximum dimension of approximately 100% to 300% of the sensingdistance.
 55. The method of claim 54 wherein providing a sensing systemfurther comprises arranging the field sensors in an array occupying anarea having a maximum dimension of approximately 200% of the sensingdistance.
 56. A method of manufacturing a system for sensing andlocalizing a wireless marker configured to wirelessly transmit a markersignal in response to a wirelessly transmitted excitation signal,comprising: selecting a sensing distance at which a sensor array is tobe positioned from the marker in operation based upon the marker signal;providing a sensing system having a support member and a plurality offield sensors carried by the support member, the field sensors beingarranged in a locally planar array relative to one another andresponsive to field components at least substantially normal to thefield sensors, and the field sensors being arranged in an array havingmaximum dimension of approximately 100% to 300% of the sensing distance;and providing instructions to position the sensing system apart from themarker in operation by a distance at least approximately equal to thesensing distance.